1. Field of the Invention
The present invention relates to image encoding technologies when an image is compressed and encoded, and more particularly, relates to an image encoding device for predicting an object to be encoded from information correlated within the image so as to encode a prediction error to thereby serve to improve encoding efficiency.
2. Description of the Related Art
There has been a method for reducing temporal redundancy and a method for reducing spatial redundancy as a method for improving encoding efficiency in the conventional image encoding.
A frame difference method or a motion compensation method is employed as a method for reducing temporal redundancy. In the frame difference method, a simple subtraction between successive two images is performed, and the obtained difference is encoded.
In the motion compensation method, a motion vector is applied to a reference frame so that an approximate image of a frame to be encoded is generated, and a difference between the approximate image and the frame to be encoded is encoded. Since the motion compensation method reduces a difference between images and encodes the difference, it is more advantageous than the frame difference method in terms of the encoding efficiency.
On the other hand, a method for estimating a motion vector used in the motion compensation method enables reduction of the temporal redundancy; however, this method has a problem that the method cannot be applied to a still image.
There is a method for quantizing an orthogonal conversion coefficient as a method for reducing spatial redundancy employed to improve the encoding efficiency. In this method, the orthogonal conversion maps a pixel signal to a frequency band and concentrates the energy into the lower band. According to this method, indiscriminating of visual characteristics by humans to the high band is used and a high band component is removed by the quantization so that the encoding efficiency can be improved.
Further, when encoding is performed, as explained in Patent Document 1, local decoding is performed on an already-encoded block around a region to be encoded to thereby generate a decoding signal. The decoding signal is used to perform an intra prediction to thereby generate a prediction signal. A predetermined orthogonal conversion and quantization are performed on a residual signal obtained by a difference between a signal component within the block to be encoded and the generated prediction signal to thereby reduce the spatial redundancy. This is generally performed to serve to improve the encoding efficiency.    Patent Document 1: Japanese Patent No. 3734492
The use of a combination of the above-described orthogonal conversion and quantization or the method for reducing spatial redundancy by the spatial prediction method can reduce the spatial redundancy within a frame; however, there is a problem that a different color signal in the same frame is independently processed, and thus, the redundancy of the color signal cannot be reduced.